Lipoprotein lipase (LPL) is a secreted lipase and the key enzyme in clearing triglycerides from circulating lipoproteins. Because elevated plasma triglycerides are an independent risk factor for cardiovascular disease, we seek ways to enhance LPL activity, which will lower plasma triglycerides. LPL activity is downregulated by a family of macromolecular inhibitors known as angiopoietin-like (ANGPTL) proteins. This family includes the LPL inhibitors ANGPTL3, ANGPTL4, and ANGPTL8. Loss-of-function mutations in the ANGPTLs results in increased LPL activity, and decreased serum triglycerides. Our objectives in this study are to better understand how the ANGPTLs associate with LPL, and with each other, so that we can target these interactions to prevent LPL inhibition. We will achieve these objectives in three specific aims. In Aim 1, we will build on our knowledge of the mechanisms used by ANGPTL3 and ANGPTL4 to inhibit LPL to identify new therapeutics that specifically block the inhibitor-LPL interaction. In Aim 2, we will use cryo-electron microscopy to solve structures of LPL bound to its inhibitors. Finally, in Aim 3, we will study the function of inhibitors within cells, and their transport out of cells. Successful completion of these aims will provide molecular details describing how LPL and its inhibitors come together, as well as new ways to block these interactions. Because of LPL's key role in regulating plasma triglyceride levels, these discoveries hold promise for new ways to prevent cardiovascular disease.